Abstract
Two ruthenium complexes [Ru(phen)~2~(m‐36)]^2+^ and [Ru(aphen)~2~(m‐36)]^2+^, indicated as 1 and 2 respectively, which contain a 36‐membered macrocycle incorporating a sterically hindered bipyridine (m‐36), and two 1,10‐phenanthroline (phen) or 4‐(p‐anisyl)‐1,10‐phenanthroline (aphen) ligands, have been synthesised. Under light irradiation, as seen from the metal‐to‐ligand‐charge‐transfer (MLCT) bands of the visible spectral region, the [Ru(phen)]~2~^2+^ core of 1 and 2 is selectively expelled from the macrocycle to give the corresponding bis‐acetonitrile derivative. This photochemical process was found to be notably less efficient than that observed in a simpler related complex [Ru(phen)~2~dmbp]^2+^ (dmbp = 6,6′dimethyl‐2,2′‐bipyridine), that lacks the macrocyclic motif. In the case of 1 thermal back reaction takes place quantitatively affording a completely reversible system. The isolation, by chromatographic techniques, of the axial‐axial cis‐[(a,a)Ru(aphen)~2~Cl~2~] isomer, where the two aphen ligands bear two p‐anisyl groups disposed trans to one another enabled the preparation of 2. This type of complex undergoes a more complicated photochemical process, in fact, following the expected photolabilisation of the [Ru(aphen)~2~]^2+^ unit, photoisomerisation of the primary photo‐product occurs and gives a statistical mixture of the three geometrical isomers: cis‐(a,a)Ru(aphen)~2~(CH~3~CN)~2~~2~, cis‐(a,e)Ru(aphen)~2~(CH~3~CN)~2~~2~ and cis‐(e,e)Ru(aphen)~2~(CH~3~CN)~2~~2~. This side‐reaction precludes its use as rotaxane precursor suggesting that, for this purpose, a bis‐phenanthroline ligand stabilising the ruthenium core is needed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)